Quick answer
Build a rain garden by testing infiltration, sizing it at 10–20% of the roof area draining to it, keeping it 10 feet from the foundation, digging a 6–12 inch flat-bottomed bowl, amending the soil, setting a stable inlet and an overflow, then planting densely.
Before you dig
1. Call 811
Every excavation starts with a utility locate. A rain garden is shallow, but gas, electric and fibre lines can be shallower than you think.
2. Test infiltration
Dig a hole the depth of the planned garden, fill it with water, and let it drain. Refill and time the drop. If the hole holds water for more than 24 hours, the site needs amendment, an underdrain or a different location.
3. Measure the roof area
Measure the footprint of the roof that drains to the downspout, not the whole roof if only one side feeds the garden. A 1,000 sq ft roof area draining to the garden is the design input.
4. Size the garden
Use 10–20% of the contributing roof area, adjusted for soil. Sandy soil can take a smaller garden; clay needs a larger one or an underdrain. For 1,000 sq ft of roof, that is 100–200 sq ft of garden.
5. Choose the location
Downslope from the downspout, at least 10 feet from the foundation, away from utilities and septic, and not in a low spot that already stays wet. The garden should receive water, not collect the whole yard’s drainage.
The build
Step 1: Mark the shape
Use a hose or string to lay out a gentle oval or kidney, wider than it is deep. Avoid a perfect circle unless the site calls for it. The edge should be level.
Step 2: Strip the sod
Remove the grass and roots from the footprint. Keep the soil you excavate if it is good; stockpile it for the berm.
Step 3: Dig the bowl
Excavate 6–12 inches, with a flat bottom and level edges. Use the spoil to build a low berm on the downslope side, leaving an overflow notch. Do not compact the bottom; scarify it to help infiltration.
Step 4: Amend the soil
Mix 2–4 inches of compost into the top 6–8 inches. If the soil is heavy clay, add coarse sand and organic matter, or install an underdrain. Do not line the garden with plastic or fabric.
Step 5: Set the inlet
Connect the downspout with a buried pipe, a gravel channel or a shallow swale. Add a rock apron at the entry to slow the flow and prevent scouring. Keep the inlet above the ponding depth so it does not back up.
Step 6: Set the overflow
Every rain garden needs a safe path for extreme storms. A graded lip on the downslope side, a rock channel or a pipe to a lower area. Never let overflow run toward the foundation.
Step 7: Plant
Plant the wet center with sedges, iris and swamp milkweed; the moist slope with coneflower, aster and joe-pye weed; the dry edge with little bluestem and butterfly weed. Plant densely and water in.
Step 8: Mulch
A thin layer of shredded hardwood mulch, 2 inches, keeping it off stems. Do not use a thick mat or landscape fabric.
After the build
- Water the plants through the first summer, especially during dry spells.
- Check the inlet and overflow after every major storm.
- Weed monthly in the first year; the plants will close the canopy.
- Replace mulch every two to three years.
- Do not fertilize; the garden is filtering runoff, and nutrients should go to the plants, not the storm drain.
A realistic weekend
A 150 sq ft garden is a two-person, two-day project with a rented tiller and a wheelbarrow. A 300 sq ft garden on clay is a three-day project and may need an underdrain. If the site is sloped or the soil does not drain, hire a landscaper who builds drainage.
Next: what it costs · design guidance.
FAQ
How deep should a rain garden be?
Usually 6–12 inches, with a flat bottom and level edges. Deeper bowls hold more water but need more excavation and take longer to drain. The depth should match the infiltration rate and the runoff volume.
Do I need a permit for a rain garden?
Usually not for a small residential garden that receives roof runoff, but check with your municipality. Grading that changes drainage, work near a wetland or a garden in a right-of-way may require a permit.
What is the best soil for a rain garden?
A sandy loam with organic matter that drains within 24–48 hours. Heavy clay needs amendment or an underdrain; pure sand drains too fast and supports few plants. A soil test and a perc test tell you what you have.